Thats Fine with the gap. The vapor barrier stops the cavity to be drowned in condensate, however residual moisture is always there from the outside air. Make sure all is perfectly air tight and celotex really snuck with timbers. Needs to properly wedged in, so in theory you should now not be...
Hi there
as far as I know, it's not a good idea to combine the different types of insulation. In Buidling Control terms it might be even illegal- someone mentioned that to me a while back (I'm an Architect).
The timber rafters will bypass the celotex and get in contact with the rockwool- not...